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September 1, 2005

Hubble Captures Neptune’s Dynamic Atmosphere

by Sam Savage

NASA -- New NASA Hubble Space Telescope images of the distant planet Neptune show a dynamic atmosphere and capture the fleeting orbits of its satellites. The images have been assembled into a time-lapse movie revealing the orbital motion of the satellites.

Images were taken in 14 different colored filters probing various altitudes in Neptune's deep atmosphere so that scientists can study the haze and clouds in detail.

These are several snapshots from the Neptune movie.

The natural-color view of Neptune (to left), common to naked eye telescopic views by amateur astronomers, reveals a cyan colored planet. Methane gas in Neptune's atmosphere absorbs most of the red sunlight hitting the planet, making it look blue-green. The image was created by combining images in red, green, and blue light.

Neptune's subtle features are more visible in the enhanced-color view (top right). Images taken in special methane filters show details not visible to the human eye (bottom right). The features seen in this enhanced image must be above most of the sunlight-absorbing methane to be detectable through these special filters.

The planet is so dark at the methane wavelengths that long exposures can be taken, revealing some of Neptune's smaller moons. Clockwise from the top (in composite image at left), these moons are Proteus (the brightest), Larissa, Despina, and Galatea. Neptune had 13 moons at last count.

Neptune is the most distant giant planet in our Solar System, orbiting the Sun every 165 years. It is so large tht nearly 60 Earths could fit inside it. A day on Neptune is between 14 hours and 19 hours.

The inner two thirds of Neptune is composed of a mixture of molten rock, water, liquid ammonia and methane. The outer third is a mixture of heated gases comprised of hydrogen, helium, water and methane.

On April 29 and 30, 2005, Hubble images were taken every 4-5 hours, spaced at about a quarter of Neptune's rotational period. These where combined to create a time-lapse movie of the dynamic planet.

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Video: Neptune's Dynamic Atmosphere -- This time-lapse movie of Neptune was assembled from combining NASA Hubble Space Telescope images taken over a 15-hour period. The movie shows a dynamic atmosphere and captures the fleeting orbits of Neptune's satellites. The natural color and enhanced color views of Neptune were assembled from images in 14 different colored filters. This allows atmospheric features to be seen above Neptune's methane haze, which gives the planet its blue-green color.

The animation has four sections, each one corresponding to approximately one rotation of Neptune.

The first section shows Neptune and its largest satellite Triton, which is about as large as our moon. Probably a captured Kuiper Belt object, Triton orbits Neptune in a "backwards" or retrograde path relative to the other major satellites, and opposite to Neptune's rotation. The view is close to what a human eye would see looking through the Hubble Space Telescope.

The second scene zooms in on Neptune, and the colors have been enhanced to better show the subtle detail of clouds in Neptune's atmosphere.

The third scene has the spectral region of light changed from the visible to special methane bands in the near infrared. Most of Neptune's atmosphere becomes very dark, except for high-altitude clouds.

The fourth scene traces the satellite orbits. Four of the small, inner satellites are visible orbiting Neptune. Dutifully obeying Kepler's laws of planetary motion, the outer moons take longer to revolve around Neptune than the inner ones. From outside to inside, these satellites are Proteus, Larissa, Galatea, and Despina. These moons are so faint that only the longest Hubble exposures can capture them.

The outermost layers of Neptune's thick atmosphere do not rotate like a solid body. The polar atmospheric regions finish a full rotation in a speedy 15 hours, while the equatorial regions lag behind, taking more than 18 hours for a complete circuit of the planet.